After seeing groups like Baseball Girls (the ones that went to Japan Expo in Paris, not the cute ones), Akamaru Dash, Pottya, etc. go major as soon as they appeared, I don't think it's completely up to popularity. After seeing Upga and Negicco go for years and still not go major, there definitely have to be some other factors at play.

At the very least, they spend the money to make some proper music videos and design unique costumes.